I don't really know. I guess it could be possible (with his schizophrenia), but if you've played the prequel then you know Rattman escaped GLaDOS's neurotoxin and since then has been in hiding and leaving messages for Chell (http://theportalwiki.com/wiki/Doug_Rattmann). Here's two theories of mine: Everything from Chell's POV is real (more likely), or Rattman made her up to cope.